1. Field of the Invention
The present invention relates generally to a multiple computer system which shares a common data bus and more particularly to a multiple computer system in which the data output circuit of at least one computer station can be disconnected in terms of hardware and software from the common data bus and checked to see if there is an abnormality within the computer unit of the computer station, so that the failure of the entire system can be prevented.
2. Description of the Prior Art
A system has been proposed in which a common data bus serves a plurality of computer stations connected in parallel for the transfer of data between different computer stations in a time-share mode.
Such a multiple computer system comprises a plurality of computer stations each having a computer unit capable of performing a sharable independent control facility, a common data bus with which each computer station is connected in parallel for transferring resultant data among the computer stations, DC power supply, power switch, and ground line.
Recently, various microcomputer systems have been mounted in automotive vehicles for electronically controlling the overall operation of the internal combustion engine, air conditioner, seat position, etc., of an automotive vehicle.
In such a multiple computer system, if an abnormality occurs during execution of a program within any of the stations, the entire system may become "hung up" since incorrect data transfer will be carried out throughout the system.
Especially, the data output circuit of each computer unit is susceptible to voltage surge from the common data line developed by other electrical apparatus, e.g., an engine ignition system, solenoid coil type valves, a motor, etc.
To protect the data output circuit from voltage surges as described above, a shielded wire is used for each signal line of the common data bus in conjunction with a surge absorber.
However, if an excessively high voltage surge is developed and applied to a computer unit, e.g., when the automotive vehicle in which such a system is mounted is struck by lightning or passes near high-voltage cable lines, such voltage surges can disturb the input or output circuit of the computer unit and accordingly the normal data transfer among the stations becomes impossible due to the hardware failure of the output circuit even though program execution is performed normally. Consequently, the entire computer system can be hung up due to a hardware defect within the output circuit of any one computer unit.